System for dynamically adjusting clipping in editing electrocardiogram waves

ABSTRACT

The disclosure sets forth a feedback system for dynamically adjusting the clipping levels in electrocardiogram analyses. The input-analog data is used to control the sampling rate in such a manner that only enough samples, including all significant deviations, are taken to describe the wave adequately. This makes it unnecessary to take a very large number of samples on a noisy record while it avoids missing important points.

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[72] Inventor David C. Alexander Yorktown Heights, N.Y. [21] Appl. No.677,685 [22] Filed Oct. 24, 1967 [45] Patented May 25, 1971 [73]Assignee International Business Machines Corporation Armonk, N.Y.

[54] SYSTEM FOR DYNAMICALLY ADJUSTING CLIPPING IN EDITINGELECTROCARDIOGRAM WAVES [56] References Cited UNITED STATES PATENTS2,497,693 2/l950 Shea 328/115 3,353,106 11/1967 Dudek etal. 307/235XPrimary ExaminerDonald D. Forrer Assistant Examiner.lohn ZazworskyAttorneys-Edwin M. Thomas, Ralph L. Thomas and Thomas & Thomas 4 Claims5 Drawing Figs ABSTRACT: The disclosure sets forth a feedback system for[1.8. CI 328/168, dynamically adjusting the clipping levels inelectrocardiogram 307/235, 307/237, 324/77, 328/1 17, 328/165, analyses.The input-analog data is used to control the sampling 328/169 rate insuch a manner that only enough samples, including all Int. Cl H03k 5/08significant deviations, are taken to describe the wave Field of Search307/235, adequately. This makes it unnecessary to take a very large 237;328/114, 1 15.7, 165, 167, 168, 169, 171; number of samples on a noisyrecord while it avoids missing 324/77 important points.

'1 CLIPPER 25 I5 "mcn' FELL "men CLIPPER WAVE INDICATION to RECTIFIER 2|COMPUTER CLIPPER W2 "MED" H 8 Eli 9" COIIPUTER DIFFERENTIATOR CLIPPERSINGLE ow' 1 SHOT .L g n I 4O n *"5 0mm; LOW AND "LOW CLIPPER PASS 28CONTROL INDICATION T0 FILTER LOGIC COMPUTER 3'! 53 S Ill G LE SHOT l lPATENTEU HAYZS 1971 SHEET 3 [1F 3 SYSTEM FOR DYNAMICALLY ADJUSTINGCLIPPING IN EDITING ELECTROCARDIOGRAM WAVES BACKGROUND OF THE INVENTIONIn recent years a number of efforts have been made to in terpretelectrocardiogram waves by electronic means such as by use of analogcomputers and/or digital type electronic data processing equipment.Because abnormalities may sometimes be widely spaced on anelectrocardiogram, it has been found necessary frequently to take longand extensive records. In some of these systems, waveforms aretransmitted in real time modes to the central data processing area wherethey are inputted to the electronic processing equipment. In some ofthese devices a group or series consisting of a large number of pulsesor waves resulting from particular phenomena, as detected by theelectrocardiogram, are utilized. Interpretation of such data in detailrequires considerable data handling.

In electrocardiograph equipment, as is well known, various and fairlynumerous types of signals are obtained. They include, firstly, theprincipal waves of the electrocardiogram which show the majorcontractions and relaxations of the heart muscles. Supplementary datamay be derived, however, including such detailed records as theelectroencephalogram (EEG), superimposed traces such as galvanic skinresponses (GSR) and the like. The electrocardiogram traces commonlyinclude also groups or series of waves or pulses which apparentlyresultfrom depolarization of the ventricles of the heart prior tocontraction. These signals are often referred to as the QRS complex.Immediately preceding each QRS series is normally a small pulse whichresults from initiation of the muscular activity and is known as the Pwave. Another signal which follows the group of pulses is commonly knownas the T wave, the latter being separated from the QRS series by asocalled ST segment.

It has previously been suggested that instead of data processing thewhole system of waves obtained in the electrocardiograph, a methodinvolving employment of a modified differentiation editing circuit maybe used. Such a system has been devised to sample the wave at points ofsignificant change points, e.g. at peaks, valleys, or at the starts andends of rises and falls in the wave form. This system has the advantagesof reducing the required data sampling rate to an average of about 20pulses or signals per second, which is a whole order of magnitude lessthan for the equal interval sampling method. Obviously, the amount ofcomputer time required is decreased proportionately. Thus data have beentaken of the P amplitude, PR level, PR interval, QRS amplitude, theaverage QRS amplitude, the QRS interval, the QT interval, the ST level,the T amplitude, as well as factors of heart rate and isoelectric level.In the system just described, the EKG data are amplified and fed to alow-pass filter to take out high frequency noise, especially 60-cyclenoise. This filter has a cutoff frequency of 30 of CPS, dropping off atabout 30 db per octave. The output of this low-pass filter is an inputto one channel of the strip recorder, which has two channels, and alsoto an editing circuit. This is a single-shot combination which producesa pulse at each significant change in the wave shape. This change" isthen an input to the other channel of the strip chart recorder. As aresult, the heart beat data were made to stand out simply and sharply,facilitating manual digitization, including pulse time and amplitude.Subsequently, the data so obtained were fed to a digital computer foranalysis. The points indicated by the editing circuit were used formaking the respective measurements. See Analysis and Pattern Recognitionof Electrocardiogram Wave Forms Using a Simulated On-Line HybridComputer System," by D. Wortzman et al., 16th Annual Conference onEngineering in Medicine and Biology, Baltimore, Md., Nov. l820, 1963(Proceedings). The system just described lacked a desirable feature inthat it had a tendency to gloss over minor higher frequency wavefluctuations which in some cased might be of significance.

SUMMARY OF THE INVENTION According to the present invention, a feedbacksystem is devised for dynamically adjusting the clipping levels ofcomparators, so as to take out noise and other unnecessary highfrequency signals. By this means, the clipping level is increased wherethe record is noisy. The high clippers are essentially unchanged ascompared to the Wortzman system described above but their primaryfunction becomes that of selecting points on high slope portions of EKGwaves, eg the QRS waves mentioned above.

Low clippers are provided also. These may respond to QRS waves but theyare intended to locate the relatively low slopes of the P and T waves.Since only the low slopes are of interest here, it is possible to reducethe noise on the signal presented to the low clippers. This is done byadding a filter which reduces the band width that is passed on.

The invention includes a single-shot and capacitor means which set thedesired low clipping levels. In the absenceof inputs, these clippinglevels are designed to drop to a minimum. Whenever a threshold iscrossed, the system permits a measured amount of charge to be added tothe corresponding capacitor from the current output of its single shot.Thus each clipping level will be raised, in this case as a logarithmicfunction of the number of points it selects.

The filters and associated elements are designed to give higher clippinglevels for noisy records and to select only large noise excursions andnoise which adds to the signal to produce a larger excursion.

Conversely, clipping levels normally will be lowest in the relativelyquiet T to P interval on the electrocardiogram. This design minimizeschances of missing the relatively small but often significant P waveitself. Since clipping levels are raised only as a function of thenumber of points picked, and not as a function of the actual wave form,clipping levels are not elevated to an objectionable degree after alarge, high slope QRS wave or wave series. Two independent circuits areused to provide ability to compensate for a small DC offset from thefilter.

A further feature is elimination of a delay circuit in the analog pathwhich was used in the prior art system. Conversions are made at auniform rate, with flag bits appended, to indicate selected points tothe computer program. With this arrangement the program makes thenecessary adjustments to compensate the delay.

The circuit of this invention edits" a time variant wave form,especially for. but not necessarily limited to electrocardiograms. Thisis accomplished by sampling the wave form only at definitive slopeconditions. The system provides two outputs in the form of pulses whichsample the raw wave form for analog-to-digital conversion to be enteredinto and analyzed by the computer. The clipper" circuits are actuallyvoltage comparators which yield a binary one output whenever variableinput voltage equals or exceeds a reference voltage.

It is one object of the invention to provide dynamic adjustment for theclipping levels of the prior art and a further object is to cut outunessential noise while retaining the capability of picking up minorsignals of significance in relatively quiet portions of the record.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of theinvention.

FIG. 2 is a graph showing how the original electrocardiogram is clippedor simplified.

FIGS. 3A and 3B are the wiring diagrams, with some components shown inblock for simplification.

FIG. 3C shows the relationship between FIGS. 3A and 3B.

- DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I thereis shown a block diagram of the system. It includes a differentiator IIwhich receives the amplified output from the amplifier 8 of theelectrocardiograph device. The system also includes an eliminationfilter unit 13, not shown in FIG. I but shown at the top of FIG. 3A.This is a diagram of a 50 HZ narrow-band elimination filter which isused to eliminate power line frequency. This particular model isdesigned for 50 cycle current, which is widely employed in Europe andother parts of the world. Inthe United States a 60 HZ unit would besubstituted. The differences are not important, however, for purposes ofthis invention. By screening out the undesirable power line frequencyfluctuations, the electrocardiogram left to be scanned is substantiallyclarified, making its recording and analysis in the computer 9 clearer.This elimination filter may precede the differentiator in the circuit.

The differentiator I1 is of a well known construction and design such asthat described on page 49 of Applications Manual for ComputingAmplifiers," published by Philbrick Researches, Inc. of Detham, Mass.The differentiator 11 is shown in detail in the lower left hand part ofFIG. 3A where the dotted line, box 11, corresponds to box ll of FIG. 1.From the differentiator the signal goes to a full-wave rectifier 15,shown also in dotted line box 15 at the bottom center of FIG. 3A. Therectifier is designed to show the rate of change in the signal voltage,regardless of direction of such change in the signal. A suitablefour-way rectifier is shown also on page 59 of the manual justmentioned. The specific design of the rectifier forms no part of thepresent invention. This device does away with the distinction betweenpositive and negative signals so that positive and negative slopes lookalike to the unit which follows. The purpose for this will be explainedfurther below.

The output signal from the differentiator also passes to a low-passfilter unit 17. This device is shown in detail in the middle left ofFIG. 3A. The circuit is so designed that a lowslope on the curve of thesignal, for example, changes slowly to let the signal through. Thislow-pass filter device is of conventional type. It is shown here as athree-slope active filter, including an operational amplifier, 18, whichembodies four resistors and three capacitors. A good example of this isdescribed in detail in Appendix 8 of Radiotelemetry by Nichols andRanch. Per se, the filter is not part of the invention but it is asignificant part of the combination.

The output from the full-wave rectifier is taken to clippers l9 and 21which are designated, respectively, "clipper-high" and clipper-medium"in FIG. 1. These are shown combined in the box 20, at the lower right ofFIG. 3A, where the circuits are given in detail. The high clipper I9 hasa high threshold and indicates to the computer 9 whether or not thisthreshold has been crossed by the signal. Similarly the medium clipper21 clips off only the signals below the medium level. The clippers arestraight comparators of a type known in the prior art. Signals from bothof these devices go to the gating and control logic 25. See also thelower part FIG. 35.

From the clippers the signal voltage is screened so that only the highor medium high portions pass on to the logic unit and are giveneventually as an indication to a load device such as the computer. Theoutput at this point gives an indication to the computer that asignificant change of some kind is coming along and thereby alerts thecomputer to record it in detail. The details to be recorded are suppliedby other parts of the circuit to be described in further detail below.

The gating and control logic just mentioned, indicated at 25, FIG. I,and in detail in the box 25 at the bottom of FIG.

3B, is simply a binary device showing that the threshold level has orhas not been crossed.

Referring next to the lower part of FIG. I, the signal from the low-passfilter I7 goes to a junction 28, from which it is compared in either apositive low-clipper or a negative lowclipper, depending on the polarityof the signal voltage. The circuits here each include a "single shot"device, which trips and resets after the signal passes. A capacitor C,and a resistor R, is included in each case. These circuits 31, 33 setthe desired low clipping levels in conjunction with single shot circuits35, 37, FIG. 1. The circuitry is shown in detail in the box 30, asalready mentioned. In the absence of any inputs from the low-pass filter17, the clipping levels drop to a minimum. Each time the threshold ofthe low-pass filter is crossed by a signal, however, a measured amountof charge, either positive or negative, is added to the appropriatecapacitor C, in the low-clipper circuit. The single shot device allowsthe pulse to go through to the capacitor, increasing the charge thereonby an appropriate increment. Thus, each clipping level will be raised anotch, so to speak, as each signal comes through. The circuitry is sodesigned that the clipping level will be raised as a logarithmicfunction of the number of points it selects. The result is that theclipping levels are automatically raised higher for noisy records andare lowered for relatively quiet records. Thus, for a noisy record, onlythe larger noise excursions are noticed, e.g. at 36, 38 in FIG. 2. Noisewhich adds to the signal to produce a still larger excursion is noticedparticularly. In other words, the capacitor involved shows a voltagewhich increases each time the threshold is crossed in the appropriatedirection, positive or negative. Conversely, clipping levels will belowest after relatively quiet parts of the electrocardiogram, e.g. fromT to P. See the discussion above regarding QRS, T, P, etc. The points Tand P as shown on the top line of FIG. 2 are quite unreliable comparedto points 36, 38 in the lower graph. Thus the chances of minimizing awave which has important implications, e.g. asmall but significant Pwave, or change in a series of P waves, are minimized. Since theclipping levels are raised only as a function of the number of pointspicked, and not in proportion to the actual wave form, the clippinglevels are not unduly elevated, even after a large high-slope portion,e.g. a QRS part of a curve.

The output of the positive and negative low clippers, with theirassociated positive and negative single-shot circuits and RC units, goesto a gating and control logic 40, shown in detail in the upper part ofFIG. 3B. The and (A) or," Single shot (SS), inverter (INV) and voltageconverter (CV) elements are all shown and their operation is quiteconventional.

The two independent circuits shown are used in order to provide abilityto compensate for a small DC offset from the filter.

An advantage of the present invention is the elimination of a delaycircuit in the analog path which hitherto was needed to compensate fordelay in the various blocks shown in FIG. 1. As a result, conversionsare made at a uniform rate with flag bits appended to indicate selectedpoints for the computer program. The program then makes adjustmentsnecessary to compensate for the delay.

The circuit described above thus edits a time variant wave form, such asan electrocardiogram (EKG) by sampling the wave form only at definitiveslope conditions. The clipper circuits are actually voltage comparatorswhich yield a binary one output when the variable input voltage equalsor exceeds the reference voltage. I

In FIG. 2 a comparison between the original electrocardiogram and theoutput signal is shown dramatically. Because of the low but variableclipping levels, both positive and negative, the numerous unnecessarysample points which otherwise would be taken on the conventionalelectrocardiogram are substantially eliminated from the input to thecomputer, greatly reducing the load thereon, by as much as a whole orderof magnitude or more. The resulting signal which goes to the computer isgreatly simplified, but significant changes are not erased in thesimplification. This makes it possible for the computer, as well as forthe visual observer, to select the points or changes in voltage, smallor large, which are truly significant, or which show a distinct changefrom normal cycles, indicating, for example, a particular problem in thepatient whose electrocardiogram is being recorded.

The outputs at the right of FIG. 1, i.e. the outputs of the two clippercircuits, are pulses which indicate to the computer points of particularinterest in the EKG shown in the upper chart of FIG. 2 for entry intoand analysis by the computer.

Although the invention has been described with particular reference toits application, apparatus-wise, to analysis of electrocardiograms (EKG)with their QRS and other components, it will be understood that itinvolves both apparatus and process aspects and that these areapplicable to other records such as electroencephelograms (EEG), andother records. This includes the component features of such records andrecordings, e.g. galvanic skin response signals (GSR) and the like.

The feedback system of the present invention dynamically adjusts theclipping levels in an earlier system devised by Wortzman (which wasdesigned to sample the wave at significant points) and makes it possibleto avoid taking an unnecessary large number of samples to be certain ofgetting all the significant samples. Only enough samples are taken todescribe the wave adequately.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What I claim is:

1. A system for analyzing a complex wave form which comprises, incombination, an amplifier for amplifying the wave form, a filterconnected to the amplifier for eliminating power fluctuations from thewave form, a differentiator connected to said filter, a full waverectifier connected to said differentiator, a first clipper and a secondclipper connected to said full wave rectifier, said first clipperserving to clip signals of relatively large amplitude, said secondclipper serving to clip signals of a relatively large amplitude which isgreater than the amplitude of signals clipped by said first clipper,

a low pass filter connected to said differentiator, a third clipper anda fourth clipper connected to said low-pass filter, a first bias circuitincluding a first resistor and a first condenser connected to said thirdclipper, a first singleshot connected to said first bias circuit, asecond bias circuit including a second resistor and a second condenserconnected to said fourth clipper, a second single-shot connected to saidsecond bias circuit,

said first clipper, said second clipper, said third clipper, and

said fourth clipper having outputs provided through control means to acomputer,

means connecting the output of said third clipper to said first singleshot, and means connecting the output of said fourth clipper to saidsecond single shot, whereby the bias level of said first and second biascircuits varies as a function of the changes of the complex waveform.

2. A system for analyzing a complex waveform which comprises, incombination, first means responsive to the complex waveform for clippingportions of the complex waveform having a relatively large amplitude,second means responsive to the complex waveform for clipping portions ofthe complex wave having relatively low amplitude, said first means andsaid second means having outputs connected to a load device,

said second means including a first clipper and a second clipper, afirst bias circuit including a first resister and a first condenserconnected to said first clipper, a first sin gle-shot connected to saidfirst bias circuit, a second bias circuit including a second resisterand a second condenser connected to said second clipper, a secondsingleshot connected to said second bias circuit, means connecting theoutput of first clipper to said first single-shot, and means connectingthe output of said second clipper to said second single-shot whereby thebias level of said first and second bias circuits varies as a functionof the changes of the complex waveform. 3. A system for analyzing acomplex waveform which comprises, in combination, first means responsiveto the complex wave for clipping portions of the complex waveform havinga relatively low amplitude, said first means includin a first clipperand a second clipper, a first bias circuit inc uding a first resisterand a first condenser connected to said first clipper, a firstsingle-shot connected to said first bias circuit, a second bias circuitincluding a second resister and a second condenser connected to saidsecond clipper, a second singleshot connected to said second biascircuit,

means connecting the output of said first clipper to said firstsingle-shot, means connecting the output of said second clipper to saidsecond single-shot whereby the bias level of said first and second biascircuits varies as a function of the changes in the complex waveform,

second means responsive to the complex waveform for clipping portions ofthe complex waveform having a relatively large amplitude, said secondmeans including a third clipper and a fourth clipper, a third biascircuit connected to said third clipper, a fourth bias circuit connectedto said fourth clipper, said third and fourth clippers serving to clipsignals of a relatively large amplitude, and said fourth clipper beingbiased by said fourth bias circuit to clip signals of a large amplitudewhich is greater than the amplitude ofsignals clipped by said thirdclipper.

4. A system for analyzing a complex waveform which comprises, incombination, a full wave rectifier responsive to the complex waveform, afirst clipper and a second clipper connected to said full waverectifier, said first clipper serving to clip signals of relativelylarge amplitude, said second clipper serving to clip signals of arelatively large amplitude which is greater than the amplitude ofsignals clipped by said first clipper,

a low-pass filter responsive to the complex waveform, a

third clipper and a fourth clipper connected to said lowpass filter, afirst bias circuit including a first resister and a first condenserconnected to said third clipper, a first single-shot connected tov saidfirst bias circuit, a second bias circuit including a second resisterand a second condenser connected to said fourth clipper, a secondsingleshot connected to said second bias circuit,

said first clipper, said second clipper, said third clipper, and

said fourth clipper having outputs provided through control means to aload device,

means connecting the output of said third clipper to said firstsingle-shot, and means connecting the output of said fourth clipper tosaid second single-shot, whereby the bias level of said first and saidbias circuits varies as a function of the changes in the complexwaveform.

1. A system for analyzing a complex wave form which comprises, incombination, an amplifier for amplifying the wave form, a filterconnected to the amplifier for eliminating power fluctuations from thewave form, a differentiator connected to said filter, a full waverectifier connected to said differentiator, a first clipper and a secondclipper connected to said full wave rectifier, said first clipperserving to clip signals of relatively large amplitude, said secondclipper serving to clip signals of a relatively large amplitude which isgreater than the amplitude of signals clipped by said first clipper, alow pass filter connected To said differentiator, a third clipper and afourth clipper connected to said low-pass filter, a first bias circuitincluding a first resistor and a first condenser connected to said thirdclipper, a first single-shot connected to said first bias circuit, asecond bias circuit including a second resistor and a second condenserconnected to said fourth clipper, a second single-shot connected to saidsecond bias circuit, said first clipper, said second clipper, said thirdclipper, and said fourth clipper having outputs provided through controlmeans to a computer, means connecting the output of said third clipperto said first single shot, and means connecting the output of saidfourth clipper to said second single shot, whereby the bias level ofsaid first and second bias circuits varies as a function of the changesof the complex waveform.
 2. A system for analyzing a complex waveformwhich comprises, in combination, first means responsive to the complexwaveform for clipping portions of the complex waveform having arelatively large amplitude, second means responsive to the complexwaveform for clipping portions of the complex wave having relatively lowamplitude, said first means and said second means having outputsconnected to a load device, said second means including a first clipperand a second clipper, a first bias circuit including a first resisterand a first condenser connected to said first clipper, a firstsingle-shot connected to said first bias circuit, a second bias circuitincluding a second resister and a second condenser connected to saidsecond clipper, a second single-shot connected to said second biascircuit, means connecting the output of first clipper to said firstsingle-shot, and means connecting the output of said second clipper tosaid second single-shot whereby the bias level of said first and secondbias circuits varies as a function of the changes of the complexwaveform.
 3. A system for analyzing a complex waveform which comprises,in combination, first means responsive to the complex wave for clippingportions of the complex waveform having a relatively low amplitude, saidfirst means including a first clipper and a second clipper, a first biascircuit including a first resister and a first condenser connected tosaid first clipper, a first single-shot connected to said first biascircuit, a second bias circuit including a second resister and a secondcondenser connected to said second clipper, a second single-shotconnected to said second bias circuit, means connecting the output ofsaid first clipper to said first single-shot, means connecting theoutput of said second clipper to said second single-shot whereby thebias level of said first and second bias circuits varies as a functionof the changes in the complex waveform, second means responsive to thecomplex waveform for clipping portions of the complex waveform having arelatively large amplitude, said second means including a third clipperand a fourth clipper, a third bias circuit connected to said thirdclipper, a fourth bias circuit connected to said fourth clipper, saidthird and fourth clippers serving to clip signals of a relatively largeamplitude, and said fourth clipper being biased by said fourth biascircuit to clip signals of a large amplitude which is greater than theamplitude of signals clipped by said third clipper.
 4. A system foranalyzing a complex waveform which comprises, in combination, a fullwave rectifier responsive to the complex waveform, a first clipper and asecond clipper connected to said full wave rectifier, said first clipperserving to clip signals of relatively large amplitude, said secondclipper serving to clip signals of a relatively large amplitude which isgreater than the amplitude of signals clipped by said first clipper, alow-pass filter responsive to the complex waveform, a third clipper anda fourth clipper connected to said low-pass filter, a first bias circuitincluding a first resister and a First condenser connected to said thirdclipper, a first single-shot connected to said first bias circuit, asecond bias circuit including a second resister and a second condenserconnected to said fourth clipper, a second single-shot connected to saidsecond bias circuit, said first clipper, said second clipper, said thirdclipper, and said fourth clipper having outputs provided through controlmeans to a load device, means connecting the output of said thirdclipper to said first single-shot, and means connecting the output ofsaid fourth clipper to said second single-shot, whereby the bias levelof said first and said bias circuits varies as a function of the changesin the complex waveform.